Discuss the reasons why fixed positioning cannot be used in HTML

Discussion on the problem that fixed positioning cannot be used in HTML

With the rapid development of the Internet, web design has become more and more complex and diverse. In web design, it is often necessary to use fixed positioning (position: fixed) to control the position of elements so that the page can achieve some special effects. However, in some cases, fixed positioning cannot be used in HTML, causing designers a lot of headaches. This article will explore the inability to use fixed positioning in HTML and provide specific code examples.

1. Floating elements cause fixed positioning to fail

In HTML, the float attribute of an element will cause the element to break away from the normal text flow, which may affect the application of the fixed positioning attribute. When an element uses the floating attribute, as long as its subsequent sibling elements also apply fixed positioning, the fixed positioning will be invalid.

Code example:

<style>
    .float-box {
        width: 200px;
        height: 200px;
        background-color: red;
        float: left;
    }

    .fixed-box {
        position: fixed;
        top: 50px;
        left: 50px;
        width: 200px;
        height: 200px;
        background-color: blue;
    }
</style>

<div class="float-box"></div>
<div class="fixed-box"></div>

In the above code example, the .float-box element has the float attribute applied, and the .fixed-box element has the float attribute applied fixed positioning attribute. However, due to the presence of floating elements, fixed positioning fails. No matter how we adjust the top and left attributes of the .fixed-box element, we cannot change its position.

Solution:

To solve this problem, you can add an empty <div> element after the floating element and give this <code><div> elements apply the <code>clear: both attribute. This can clear the impact of floating elements and ensure that subsequent fixed-positioned elements are displayed normally.

Code example:

<style>
    .float-box {
        width: 200px;
        height: 200px;
        background-color: red;
        float: left;
    }

    .fixed-box {
        position: fixed;
        top: 50px;
        left: 50px;
        width: 200px;
        height: 200px;
        background-color: blue;
    }

    .clear-fix {
        clear: both;
    }
</style>

<div class="float-box"></div>
<div class="clear-fix"></div>
<div class="fixed-box"></div>

2. The restriction of the containing block causes the fixed positioning to fail

In HTML, the position of the fixed positioning element is based on its containing block (containing block) calculate. The containing block is the nearest positioned ancestor of a fixedly positioned element, which can be any element with a positioning attribute (position: relative, position: fixed or position: absolute) element, or the initial containing block of the document. Restrictions on containing blocks may cause fixed positioning to fail.

Code example:

<style>
    .parent-box {
        position: relative;
        width: 300px;
        height: 300px;
        background-color: yellow;
    }

    .fixed-box {
        position: fixed;
        top: 50px;
        left: 50px;
        width: 200px;
        height: 200px;
        background-color: blue;
    }
</style>

<div class="parent-box">
    <div class="fixed-box"></div>
</div>

In the above code example, the .parent-box element is an ancestor element with a positioning attribute. However, since the .parent-box element itself is also a block-level element, the containing block of the fixed-positioned element .fixed-box is restricted to inside .parent-box . This means that the fixed positioning of the .fixed-box element may only apply to the display area of ​​.parent-box, and not beyond this range.

Solution:

To solve this problem, you can create a new positioned element as a containing block outside the .parent-box element to ensure that the positioned element is fixed The position calculation is relative to the entire document. This removes the restriction of the containing block and allows fixed positioning to take effect.

Code example:

<style>
    .parent-box {
        width: 300px;
        height: 300px;
        background-color: yellow;
    }

    .fixed-box {
        position: fixed;
        top: 50px;
        left: 50px;
        width: 200px;
        height: 200px;
        background-color: blue;
    }

    .fixed-container {
        position: relative;
    }
</style>

<div class="fixed-container">
    <div class="parent-box">
        <div class="fixed-box"></div>
    </div>
</div>

By applying the position: relative positioning attribute on the .fixed-container element, we create a new containing block, making the containing block of the fixed-positioned element .fixed-box become the entire document. In this way, the fixed positioning of the .fixed-box element can take effect normally.

To sum up, the problems that fixed positioning cannot be used in HTML mainly include fixed positioning failure due to floating elements and the limitation of containing blocks. By appropriately adjusting the HTML structure and style, we can solve these problems and ensure that the application of fixed positioning attributes takes effect normally.